Y'all have seen this type of double-confirmation popup, I'm sure:
It's helpful if you want to give end users a little additional nudge, for example to use their legal name, a working email address, and so on — stuff that you can't force them to do but maybe can guilt them into.☺
No one but me would claim it's “easy” to add this functionality to a Marketo form. It only took an hour to whip it up, but as you know, I
waste spend more time in Forms 2.0 API-land than anybody else. Lucky for you, I've provided a big leg up by linking to a working CodePen that you can spin off for your own site.